The field of the present invention is separators employing vibratory screens.
Vibratory screen systems have long been employed for the separation of solids suspended in liquid as well as solids of various sizes. The screens are generally drawn taut, oriented in a roughly horizontal position and vibrated in such a way that material will move advantageously across the screen during the screening process. Such vibratory structures are often characterized as either being rectangular or circular, each exhibiting its own advantages, motions design features and difficulties.
Two difficulties encountered with rectangular screens have been screen sealing and the avoidance of whipping. Screen sealing is necessary to prevent bypass, a condition where material on the screen is able to pass around the frame to again comtaminate the already screened material. Whipping is a condition where the center, unsupported area of the screen is able to vibrate at a greater amplitude than the frame. This may result from a transitory or continuous condition of partial resonance. Such a condition is disadvantageous because the material on the screen does not experience sufficient residence time for proper screening. Additionally, the material is not efficiently transported across the screen under such conditions and blinding from oversize material can occur.
The foregoing problems of bypass and screen whipping are brought to the fore because solutions to each of these problems in a rectangular screen have been mutually exclusive to any satisfactory degree. To effect proper sealing, prefabricated and pretensioned screen assemblies have been found most useful. The rigid frame structure may be easily pressed against a seal about all sides to eliminate bypass. Such rigid screen assemblies have found additional advantage through the use of known inflatable pneumatic tubes employed as the sealing mechanism. Through controlled deflation of the tubes, a sreen structure may be easily placed or removed. Inflation of the seal then properly locks and seals the screen in position. However, in large rectangular screening mechanisms whipping becomes a problem for such pretensioned screen structures regardless of the sealing mechanism. Because of the difficulties of tensioning and fabrication, structural bowing of the screen and the like (a known aid against whipping) has not been found practical.
To solve the whipping of large rectangular screens, stationary stays have been introduced into the vibratory frame which are bowed or crowned. Such a construction has required post tensioning of the screen which is laid over the frame and then pulled tight on two sides. This mechanism may substantially eliminate whipping but provides a less than satisfactory seal about the edge of the screen. With certain products such as thin coating material, any oversized particles bypassed to the final product cannot be tolerated.